In order to improve the performance and efficiency of a gas turbine engine it has been necessary to reduce the amount of air used for cooling the turbine components. This has lead to the replacement of air cooled metallic shroud segments with uncooled ceramic matrix composite shroud segments, for example silicon carbide fibres in an alumina matrix.
Conventional abradable seals on cooled metallic shrouds comprise metallic honeycombs. These are not suitable for use on ceramic matrix composite shroud segments. Ceramic honeycombs have been applied to ceramic matrix shroud segments to form abradable seals, but these were found to cause unacceptable wear of the nickel base turbine blades. Ceramic foams have been applied to ceramic matrix shroud segments to form abradable seals, and these did not produce significant wear on the nickel base turbine blades, however these did not provide a satisfactory seal and they suffer from erosion.